This invention relates to a method of controlling transmission power in a two-terminal D.C. power transmission system wherein two A.C. systems are associated by a direct current. More specifically, it relates to a control apparatus for a D.C. power transmission system wherein one of converters is a line-commutated converter and the other is a forced-commutated converter.
A D.C. power transmission system to which this invention is applied is constructed as shown in FIG. 1. Referring to the figure, numeral 1 designates a first A.C. system; numeral 2 designates a second A.C. system, and numeral 3 designates a line-commutated converter which is connected to the first A.C. system 1 and which changes alternating current into direct current. Shown at numeral 4 is a force-commutated converter which has a commutation means therein, and which is connected to the second A.C. system 2 through a reactance 5. Numeral 6 indicates a D.C. power transmission line which is connected between the D.C. terminal of the line-commutated converter 3 and that of the forced-commutated converter 4, and numeral 7 designates a smoothing capacitor which is connected to the D.C. terminals of the forced-commutated converter 4.
As a method of controlling the D.C. power transmission system, there has been known one method relying on the control means 13 to 15. Numeral 10 denotes a voltage detector which detects a D.C. voltage across the smoothing capacitor 7, numeral 11 designates a voltage reference generator which provides a reference voltage, and numeral 12 designates a voltage controller which takes the difference between the reference voltage and the detected D.C. voltage and which changes the ignition pulse phase of the line-commutated converter 3 on the basis of deviations in the difference and thus controls same so as to render the deviations equal to zero. The control means 13 is a power detector which detects the effective power that is exchanged between the second A.C. system 2 and the forced-commutated converter 4 through the reactance 5, the control means 14 is a power reference generator which provides a reference of the transmission power of the D.C. power transmission system, and the control means 15 is a power controller which takes the difference between the reference power and the effective power and which changes the ignition pulse phase of the forced-commutated converter 4 on the basis of the difference and thus controls same so as to render the difference equal to zero. In operation, when the reference voltage has been provided by the voltage reference generator 11, it is compared with the D.C. voltage detected by the voltage detector 10, and the voltage controller 12 changes the control angle of the ignition pulse of the line-commutated converter 3. When the D.C. voltage is lower, the control angle is advanced, and when it is higher, the control angle is retarded, whereby the voltage between the D.C. terminals of the line-commutated converter 3 is raised or lowered. As a result, charges are stored into or discharged from the smoothing capacitor 7 through the D.C. power transmission lines 6, and the D.C. voltage is controlled so as to agree with the voltage reference. On the other hand, the forced-commutated converter 4 changes this D.C. voltage into an alternating current. A voltage of a magnitude proportional to the D.C. voltage is generated at the A.C. terminal of the converter 4, and its phase is determined by the ignition pulse phase of this converter. Subsequently, when the reference power has been provided by the power reference generator 14, it is compared with the effective power detected by the power detector 13, and the power controller 15 changes the phase of the ignition pulse of the forced-commutated converter 4. When the power detected by the power detector 13 is smaller than the reference, the phase of the ignition pulse is advanced. As a result, the phase of a voltage occurring at the A.C. terminal of the converter 4 changes into a phase leading over the voltage phase of the A.C. system 2, and the phase angle difference increases the quantity of power to flow to the A.C. system 2 through the reactance 5. Conversely, when the power detected by the power detector 13 is greater than the reference, the phase of the ignition pulse is retarded. As a result, the phase of a voltage occurring at the A.C. terminal of the converter 4 changes into a phase lagging behind the voltage phase of the A.C. system 2, and the phase angle difference decreases the quantity of power. Thus, the transmission power is controlled so as to become equal to the power reference.
With the control method stated above, after the voltage of the smoothing capacitor 7 has been detected by the voltage detector 10, this information needs to be transmitted to the control part for the converter 3 on the first A.C. system side. Furthermore, since the two converters 3 and 4 are actually located a long distance apart, there is the disadvantage that the controllability is worsened by a transmission lag and noise in the course of transmission. Besides, since the voltage of the smoothing capacitor 7 is regulated by changing the voltage of the converter 3, there is the disadvantage that a control lag is incurred by the inductance of the transmission lines existing midway, resulting in a slow response.